After episode 17, I think I might have some things figured out.
In episode three, we find out Satsuki’s father called Junketsu her “wedding dress”.
Moving on to this episode, we find out she changed on that day. It also seems like by the time she had confronted Nonon when they were in Elementary school (Kindergarten, maybe?) she already had some sort of goal in mind.
Personally, I think Satsuki’s father found out about Ragyo’s insane plans about life fibers, and started to plot against her, which included telling Satsuki about her mother’s plans. He may have even been secretly in cahoots with Isshin Matoi. It seems like Satsuki and Papa Kiryuin were pretty close, so this makes sense.
Also, Satsuki specifically says “this all started on that day."
This meaning her fight against Life Fibers.
Which leads me to believe that her open rebellion against Life Fibers and Ragyo is her “wedding”.
And Junketsu is her “wedding dress” because it is the tool Satsuki will use in her “wedding” to complete it, or in her case, achieve her goals,
After all, the wedding dress is the final thing you put on for the main event.

And in order to combat life fibers effectively, you have to don them yourself. (as seen with Ryuko wearing Senketsu and pretty much pwning every single life-fiber clad enemy that comes her way)
Much like Senketsu was made to fight Life Fibers, Junketsu was secretly to be used for that purpose as well.
Junketsu was probably made at first to further Ragyo’s plans, with the sole purpose of Satsuki being used as a tool. I think Papa Kiryuin changed that when he revealed Ragyo’s true intentions to five year old Satsuki.
Instead of being used for evil, Junketsu would be used for good and Satsuki would secretly fight for her father, not her mother, hence the Kamui’s name “purity”, because even if it is composed of the very life fibers Satsuki is fighting against, the intentions for the outfit are completely pure in nature.
And the reason Junketsu is so feral, is because it knows it’s going to be used to combat its own brethren after Satsuki’s big reveal. But unlike Senketsu, Junketsu is fiercely loyal to its own kind, and as such, is actively trying to take Satsuki down.